Hydraulic system in work machine

ABSTRACT

A hydraulic system in a work machine includes a hydraulic pump to supply pilot hydraulic oil. A work hydraulic actuator is to move a work device of the work machine. A control valve is to control the work hydraulic actuator based on a pilot pressure of the pilot hydraulic oil. A target pilot pressure of the pilot hydraulic oil is input through the work operation device to control, according to the target pilot pressure, the pilot pressure supplied from the hydraulic pump to the control valve. A pilot oil path connects the work operation device and the control valve to supply the pilot pressure to the control valve. A drain oil path is divided from the pilot oil path. A pressure adjustment valve is provided in the drain oil path to adjust the pilot pressure to be less than the target pilot pressure when a condition is satisfied.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of the U.S. patentapplication Ser. No. 16/012,761 filed Jun. 20, 2018, which claimspriority under 35 U. S. C. § 119 to Japanese Patent Application No.2017-193603, filed Oct. 3, 2017, and Japanese Patent Application No.2017-193604, filed Oct. 3, 2017. The contents of these applications areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a hydraulic system in a work machine.

Discussion of the Background

In the conventional work machine, for example, the output of a hydraulicinstrument needs to be reduced because of various reasons. For example,in the disclosure of Japanese Patent No. 5687970, when an enginereceives a load equal to or larger than a predetermined load, the outputof a travel pump that is a hydraulic instrument is reduced.Specifically, a work machine disclosed in Japanese Patent No. 5687970includes an engine, a travel pump driven by the engine, a traveloperation lever, an operation valve capable of changing the pressure(pilot pressure) of pilot oil in accordance with an operation on thetravel operation lever, and a pressure control valve provided upstreamrelative to the operation valve.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a hydraulic system ina work machine includes a hydraulic pump, a work hydraulic actuator, acontrol valve, a work operation device, a pilot oil path, a drain oilpath, and a pressure adjustment valve. The hydraulic pump is to supplypilot hydraulic oil in a hydraulic oil reservoir. The work hydraulicactuator is to move a work device of the work machine. The control valveis to control the work hydraulic actuator based on a pilot pressure ofthe pilot hydraulic oil. A target pilot pressure of the pilot hydraulicoil is input through the work operation device to control, according tothe target pilot pressure, the pilot pressure supplied from thehydraulic pump to the control valve. The pilot oil path connects thework operation device and the control valve to supply the pilot pressureto the control valve. The drain oil path is divided from the pilot oilpath. The pressure adjustment valve is provided in the drain oil path toadjust the pilot pressure to be less than the target pilot pressure whena condition is satisfied.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

FIG. 1 is a view of a hydraulic system of a traveling system.

FIG. 2A is a graph of a relationship between an operation amount andpilot pressure.

FIG. 2B is another graph of a relationship between an operation amountand pilot pressure.

FIG. 3A is a view of a hydraulic system of a traveling system, accordingto a first modification.

FIG. 3B is a view of a hydraulic system of a traveling system, accordingto a second modification.

FIG. 3C is a view of a hydraulic system of a traveling system, accordingto a third modification.

FIG. 3D is a view of a hydraulic system of a traveling system, accordingto a fourth modification.

FIG. 4 is a side view of a track loader that is an example of a workmachine.

FIG. 5 is a view of a hydraulic system of a work system.

FIG. 6A is a graph of a relationship between an operation amount andpilot pressure.

FIG. 6B is another graph of a relationship between an operation amountand pilot pressure.

FIG. 7A is a view of a hydraulic system of a work system, according to afirst modification.

FIG. 7B is a view of a hydraulic system of a work system, according to asecond modification.

FIG. 7C is a view of a hydraulic system of a work system, according to athird modification.

FIG. 7D is a view of a hydraulic system of a work system, according to afourth modification.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

Embodiments which are examples of the present invention will bedescribed below with reference to the drawings.

FIG. 4 shows a side view of a work machine according to the embodimentof the present invention. FIG. 4 illustrates a compact track loader asan exemplary work machine. However, the work machine according to theembodiment of the present invention is not limited to a compact trackloader, but may be, for example, another kind of loader work machinesuch as a skid steer loader. Alternatively, the work machine may be awork machine other than a loader work machine.

As illustrated in FIG. 4, a work machine 1 includes a machine body 2, acabin 3, a work device 4, and travel devices 5. In the embodiments ofthe present invention, it is assumed that a front side (left side inFIG. 4) of a driver sitting on a driver's seat 8 of the work machine 1is a forward direction, a back side (right side in FIG. 4) of the driveris a backward direction, a left side (near side in FIG. 4) of the driveris a leftward direction, and a right side (far side in FIG. 4) of thedriver is a rightward direction. Furthermore, it is assumed that ahorizontal direction that is orthogonal to the forward and backwarddirections is a machine body width direction. Furthermore, it is assumedthat a rightward or leftward direction from a central part of themachine body 2 is a machine body outward direction. In other words, themachine body outward direction is the machine body width direction andis a direction away from the machine body 2. Furthermore, it is assumedthat a direction opposite to the machine body outward direction is amachine body inward direction. In other words, the body inward directionis a direction toward the machine body 2 along the body width direction.

The cabin 3 is mounted on the machine body 2. The cabin 3 is providedwith the driver's seat 8. The work device 4 is mounted on the machinebody 2. The travel devices 5 are provided outside of the machine body 2.An engine 9 (See FIG. 2) is mounted on a back part in the machine body2.

The work device 4 includes booms 10, a work tool 11, lift links 12,control links 13, boom cylinders 14, and bucket cylinders 15.

The booms 10 are provided swingably in the vertical direction on theright side and the left side of the cabin 3. The work tool 11 is, forexample, a bucket, and the bucket 11 is provided to leading end parts(front end parts) of the booms 10 to be swingable in the verticaldirection. The lift links 12 and the control links 13 support base parts(back parts) of the booms 10 so that the booms 10 are swingable in thevertical direction. The boom cylinders 14 extend and contract to liftand lower the booms 10. The bucket cylinders 15 extend and contract toswing the bucket 11.

Front parts of the booms 10 on the left side and the right side arecoupled with each other through a curved and forked coupling pipe. Thebase parts (back parts) of the booms 10 are coupled with each otherthrough a circular coupling pipe.

The lift links 12, the control links 13, and the boom cylinders 14 areprovided on the left side and the right side of the machine body 2 in amanner corresponding to the booms 10 on the left side and the rightside.

The lift links 12 are vertically provided to back parts of the baseparts of the booms 10. Upper parts (one end side) of the lift links 12are pivoted rotatably about the horizontal axis closer to the back partsof the base parts of the booms 10 through a pivotal shaft. Lower parts(other end side) of the lift links 12 are pivoted rotatably about thehorizontal axis closer to the back part of the machine body 2 through apivotal shaft.

Upper parts of the boom cylinders 14 are pivoted rotatably about thehorizontal axis through a pivotal shaft. Lower parts of the boomcylinders 14 are pivoted rotatably about the horizontal axis through apivotal shaft.

The control links 13 are provided in front of the lift links 12. Oneends of the control links 13 are pivoted rotatably about the horizontalaxis through a pivotal shaft. The other ends of the control links 13 arepivoted rotatably about the horizontal axis through a pivotal shaft.

The booms 10 vertically swing about the pivotal shaft through expansionand contraction of the boom cylinders 14 while the base parts of thebooms 10 are supported by the lift links 12 and the control links 13,and the leading end parts of the booms 10 move up and down. The controllinks 13 vertically swing about the pivotal shaft along with thevertical swing of the booms 10. The lift links 12 swing in thefront-back direction about the pivotal shaft along with the verticalswing of the control links 13.

Instead of the bucket 11, another work tool is attachable to the frontparts of the booms 10. Examples of the other work tool includeattachments (auxiliary attachments) such as a hydraulic crusher, ahydraulic breaker, an angle broom, an earth auger, a pallet folk, asweeper, a mower, and a snow blower.

A connecting member is provided to the front part of the boom 10 on theleft side. The connecting member is a device configured to connect ahydraulic instrument provided to an auxiliary attachment, and a firstpipe member such as a pipe provided to the boom 10. Specifically, oneend of the connecting member is connectable to the first pipe member,and the other end is connectable to a second pipe member connected withthe hydraulic instrument of the auxiliary attachment. With thisconfiguration, hydraulic oil flowing through the first pipe memberpasses through the second pipe member before being supplied to thehydraulic instrument.

The bucket cylinders 15 are arranged closer to the front parts of thebooms 10. The bucket 11 is swung through expansion and contraction ofthe bucket cylinders 15.

The travel devices 5 on the left side and the right side are crawlertravel devices (including semi-crawler travel devices) in the presentembodiment. The travel devices 5 may be wheeled travel devices providedwith front and rear wheels.

As illustrated in FIG. 1, the hydraulic system includes a firsthydraulic pump Pu1 and a second hydraulic pump Pu2. The first hydraulicpump Pu1 is used to drive a hydraulic actuator of the boom cylinders 14,the bucket cylinders 15, or an attachment attached to leading ends ofthe booms 22. The second hydraulic pump Pu2 (pilot pump) is mainly usedto supply pressure of the hydraulic oil, which is to be used as controlpressure or signal pressure. Hereinafter, for descriptive purposes, thehydraulic oil for supplying control pressure or signal pressure will bereferred to as “pilot oil”, while pressure of the pilot oil will bereferred to as “pilot pressure”.

As illustrated in FIG. 1, a hydraulic system of a traveling system is asystem configured to drive the travel devices 5.

The symbols “X1” and “X2” illustrated in FIG. 1 denote targets to becoupled to oil paths.

The travel devices 5 include a first travel motor device 35L and asecond travel motor device 35R.

A supply oil path 40 is provided to the first hydraulic pump Pu1 so thatthe hydraulic oil (pilot oil) flows therethrough. The supply oil path 40is provided with a direction switching valve 29, the first travel motordevice 35L, and the second travel motor device 35R. The supply oil path40 is further provided with a charge oil path 41.

The direction switching valve 29 is an electromagnetic valve configuredto change rotation of the first travel motor device 35L and the secondtravel motor device 35R, and is a two-position switching valveswitchable between a first position 29 a and a second position 29 bthrough excitation. A switching operation of the direction switchingvalve 29 is performed by, for example, an operation member (notillustrated).

The first travel motor device 35L is a device configured to transferpower to a drive shaft (a shaft) 5 aL of the travel device 5 provided onthe left side of the machine body 2. The second travel motor device 35Ris a device configured to transfer power to a drive shaft (a shaft) 5 aRof the travel device provided on the right side of the machine body 2.

The first travel motor device 35L includes a travel motor 36L, a swashplate switching cylinder 37, and a travel control valve (hydraulicswitching valve) 38. The travel motor 36L is a swash-platevariable-displacement axial motor capable of changing a vehicle speed(rotation) to the first or second speed. In other words, the travelmotor 36L is a motor capable of changing a driving force of the workmachine 1.

The swash plate switching cylinder 37 is a cylinder configured to changethe angle of a swash plate of the travel motor 36L through expansion andcontraction. The travel control valve 38 is a valve configured to expandand contract the swash plate switching cylinder 37 toward one end or theother end, and is a two-position switching valve switchable between afirst position 38 a and a second position 38 b. A switching operation ofthe travel control valve 38 is performed by the direction switchingvalve 29 connected with the travel control valve 38 and positionedupstream.

As described above, according to the first travel motor device 35L, whenthe direction switching valve 29 is switched to the first position 29 athrough an operation of the operation member, the pilot oil is drainedfrom a section between the direction switching valve 29 and the travelcontrol valve 38, and the travel control valve 38 is switched to thefirst position 38 a. As a result, the swash plate switching cylinder 37is contracted to set the travel motor 36L to the first speed. When thedirection switching valve 29 is switched to the second position 29 bthrough an operation of the operation member, the pilot oil is suppliedto the travel control valve 38 through the direction switching valve 29,and the travel control valve 38 is switched to the second position 38 b.As a result, the swash plate switching cylinder 37 is extended to setthe travel motor 36L to the second speed.

The second travel motor device 35R is actuated in a manner similar tothat of the first travel motor device 35L. The second travel motordevice 35R is configured and actuated in a manner similar to those ofthe first travel motor device 35L, and thus description thereof will beomitted.

The hydraulic system of the traveling system includes a plurality oftravel pumps 50. The travel pumps 50 include a travel pump 50A and atravel pump 50B. The travel pump 50A is configured to control the travelmotor 36L in the first travel motor device 35L. The travel pump 50B isconfigured to control the travel motor 36R in the second travel motordevice 35R.

The travel pumps 50A and 50B are swash-plate variable-displacement axialpumps. The travel pump 50A includes a first pressure receiving portion51 a to which the pilot pressure acts, and a second pressure receivingportion 51 b to which the pilot pressure also acts. When the pilotpressure acts onto the pressure receiving portions 51 a and 51 b of thetravel pump 50A, the angle of the swash plate of the travel pump 50Achanges. Changing the angle of the swash plate can change the outputs(supply amounts of hydraulic oil) of the travel pump 50A and thedirection of supplying the hydraulic oil.

The travel pump 50B includes a first pressure receiving portion 52 a towhich the pilot pressure acts, and a second pressure receiving portion52 b to which the pilot pressure also acts. When the pilot pressure actsonto the pressure receiving portions 52 a and 52 b of the travel pump50B, the angle of the swash plate of the travel pump 50B changes.Changing the angle of the swash plate can change the outputs (supplyamounts of hydraulic oil) of the travel pump 50B and the direction ofsupplying the hydraulic oil.

The travel pump 50A and the travel motor 36L are coupled by a speedchanging oil path 28 h so that the hydraulic oil (pilot oil) suppliedfrom the travel pump 50A is supplied to the travel motor 36L. The travelpump 50B and the travel motor 36R are coupled by a speed changing oilpath 28 i so that the hydraulic oil (pilot oil) supplied from the travelpump 50B is supplied to the travel motor 36R. The speed changing oilpath 28 h and the speed changing oil path 28 i are coupled to the chargeoil path 41 so as to supply the pilot oil to the charge oil path 41.

The hydraulic system of the traveling system includes a plurality oftravel operation devices 53. The plurality of travel operation devices53 are devices configured to respectively operate the plurality oftravel pumps 50. When operated, the travel operation devices 53 canrespectively set pilot pressure acting onto the pressure receivingportions (first pressure receiving portions 51 a and 52 a and secondpressure receiving portions 51 b and 52 b) of the travel pumps 50. Theplurality of travel operation devices 53 include a travel operationdevice 53A disposed to the left of the driver's seat 8, and a traveloperation device 53B disposed to the right of the driver's seat 8.

The travel operation device 53A includes an operation member 54A, and aplurality of operation valves 46A and 46B. The operation member 54A isformed into a lever, for example, and is configured to swing frontwardand backward. The plurality of operation valves 46A and 46B are coupled,via oil paths, to the second hydraulic pump Pu2, and are each configuredto set pilot pressure as the operation member 54A is operated.

For example, when the operation member 54A is tilted forward, theoperation valve 46A sets pilot pressure acting onto the first pressurereceiving portion 51 a in accordance with an operation amount of theoperation member 54A. When the pilot pressure acting onto the firstpressure receiving portion 51 a is equal to or above a predeterminedvalue, the angle of the swash plate of the travel pump 50A changes, andthe travel pump 50A normal-rotates.

When the operation member 54A is tilted backward, the operation valve46B sets pilot pressure acting onto the second pressure receivingportion 51 b in accordance with an operation amount of the operationmember 54A. When the pilot pressure acting onto the second pressurereceiving portion 51 b is equal to or above a predetermined value, theangle of the swash plate of the travel pump 50A changes, and the travelpump 50A reverse-rotates.

The travel operation device 53B includes an operation member 54B, and aplurality of operation valves 46C and 46D. The operation member 54B isformed into a lever, for example, and is configured to swing forward andbackward. The plurality of operation valves 46C and 46D are coupled, viaoil paths, to the second hydraulic pump Put, and are each configured toset pilot pressure in accordance with an operation of the operationmember 54B.

For example, when the operation member 54B is tilted forward, theoperation valve 46C sets pilot pressure acting onto the first pressurereceiving portion 52 a in accordance with an operation amount of theoperation member 54B. When the pilot pressure acting onto the firstpressure receiving portion 52 a is equal to or above a predeterminedvalue, the angle of the swash plate of the travel pump 50B changes, andthe travel pump 50B normal-rotates.

When the operation member 54B is tilted backward, the operation valve46D sets pilot pressure acting onto the second pressure receivingportion 52 b in accordance with an operation amount of the operationmember 54B. When the pilot pressure acting onto the second pressurereceiving portion 52 b is equal to or above a predetermined value, theangle of the swash plate of the travel pump 50B changes, and the travelpump 50B reverse-rotates.

The hydraulic system of the traveling system includes a plurality ofpilot oil paths 45. The plurality of pilot oil paths 45 are oil pathsrespectively coupling the travel operation device 53 and the pluralityof pressure receiving portions (first pressure receiving portions 51 aand 52 a and second pressure receiving portions 51 b and 52 b).Specifically, the plurality of pilot oil paths 45 include a pilot oilpath 45 a coupling the operation valve 46A and the first pressurereceiving portion 51 a, a pilot oil path 45 b coupling the operationvalve 46B and the second pressure receiving portion 51 b, a pilot oilpath 45 c coupling the operation valve 46C and the first pressurereceiving portion 52 a, and a pilot oil path 45 d coupling the operationvalve 46D and the second pressure receiving portion 52 b.

The hydraulic system of the traveling system includes a plurality ofdrain oil paths 91. The plurality of drain oil paths 91 are oil pathsrespectively coupled to the plurality of pilot oil paths 45, and areeach configured to drain the pilot oil in the plurality of pilot oilpaths 45 to a hydraulic oil tank 23 (a hydraulic oil reservoir 23), forexample. Specifically, the plurality of drain oil paths 91 include adrain oil path 91 a coupled to the pilot oil path 45 a, a drain oil path91 b coupled to the pilot oil path 45 b, a drain oil path 91 c coupledto the pilot oil path 45 c, and a drain oil path 91 d coupled to thepilot oil path 45 d.

In the plurality of pilot oil paths 45, throttle portions 93 a, 93 b, 93c, and 93 d are respectively disposed between the plurality of operationvalves 46A, 46B, 46C, and 46D and coupling portions 92 a, 92 b, 92 c,and 92 d respectively coupling the plurality of pilot oil paths 45 andthe plurality of drain oil paths 91.

The hydraulic system of the traveling system includes a plurality ofalteration portions (pressure adjustment valves) 94, and a plurality ofdifferential pressure generation portions (differential pressuregeneration valves) 95. The plurality of alteration portions 94 are eachconfigured to alter the pilot pressure (secondary pilot pressure) actingonto the pressure receiving portions (first pressure receiving portions51 a and 52 a and second pressure receiving portion 51 b and 52 b) withrespect to an operation amount of each of the travel operation devices53, for example, when a load of the engine 9 is higher than a loadthreshold. In other words, the plurality of alteration portions 94 areeach configured to at least alter an increasing rate (gradient) of thesecondary pilot pressure with respect to an operation amount of each ofthe travel operation devices 53. The plurality of alteration portions 94may each be configured to alter a decreasing rate (gradient) of thepilot pressure with respect to an operation amount of each of the traveloperation devices 53.

The plurality of alteration portions 94 are respectively disposed in theplurality of drain oil paths 91. The plurality of alteration portions 94include an alteration portion 94 a disposed in the drain oil path 91 a,an alteration portion 94 b disposed in the drain oil path 91 a, analteration portion 94 c disposed in the drain oil path 91 c, and analteration portion 94 d disposed in the drain oil path 91 d.

The alteration portions 94 a, 94 b, 94 c, and 94 d are electromagneticvariable throttle valves (electric variable throttle valves). Thevariable throttle valves are valves each configured to switch between aclosed position 96, at which a throttle area is reduced to zero, thatis, the path is fully closed, and an open position 97, at which the pathis fully open. The variable throttle valves are each configured to set athrottle area at a desired position between the closed position 96 andthe open position 97.

A throttle amount of each of the variable throttle valves can beadjusted with a switch 166 disposed on a controller 165, for example.The switch 166 is disposed adjacent to the driver's seat 8 so as to beoperated by a driver, for example. For example, the switch 166 is aswitch configured to switch between ON and OFF. A signal indicative ofON or OFF of the switch 166 then is input to the controller 165. Whenthe switch 166 is in ON, the pilot pressure in the plurality of pilotoil paths 45 (secondary pilot pressure in the operation valves 46A, 46B,46C, and 46D) is limited, while, when the switch 166 is in OFF, nosecondary pilot pressure is limited. For example, when the switch 166 isin ON, the controller 165 sets throttle amounts for the variablethrottle valves so that throttle diameters (throttle areas) of thealteration portions 94 and throttle diameters of the throttle portions93 a, 93 b, 93 c, and 93 d respectively match each other. On the otherhand, when the switch 166 is in OFF, the controller 165 causes thevariable throttle valves to each move to the closed position 96. Bydisposing an alteration operation member 168, which is a rotatablevolume or a slide switch, for example, on the controller 165, and byoperating the alteration operation member 168, an increasing rate(gradient) of secondary pilot pressure may be altered with respect to anoperation amount of each of the travel operation devices 53. The abovedescribed throttle amounts set in the variable throttle valves aremerely examples. The embodiment of the present invention is not limitedto the throttle amounts.

The plurality of differential pressure generation portions 95 arerespectively disposed in the plurality of drain oil paths 91. Theplurality of differential pressure generation portions 95 are portionseach configured to generate differential pressure upstream or downstreamrelative to the plurality of alteration portions 94.

The plurality of differential pressure generation portions 95 arerespectively disposed downstream relative to the plurality of alterationportions 94. The plurality of differential pressure generation portions95 include a differential pressure generation portion 95A disposeddownstream relative to the alteration portion 94 a, a differentialpressure generation portion 95B disposed downstream relative to thealteration portion 94 b, a differential pressure generation portion 95Cdisposed downstream relative to the alteration portion 94 c, and adifferential pressure generation portion 95D disposed downstreamrelative to the alteration portion 94 d. The differential pressuregeneration portions 95A, 95B, 95C, and 95D are check valves eachconfigured to permit the hydraulic oil to flow from the pilot oil paths45 to the alteration portions 94, as well as to prevent the hydraulicoil from flowing to the pilot oil paths 45. Although, in the embodiment,the plurality of differential pressure generation portions 95 arerespectively disposed downstream relative to the plurality of alterationportions 94, the plurality of differential pressure generation portions95 may respectively be disposed upstream relative to the plurality ofalteration portions 94.

In the hydraulic system of the traveling system, when the operationmember 54A is swung forward or backward, the travel pump 50A (travelmotor 36L) can operate smoothly. Alternatively, when the operationmember 54B is swung forward or backward, the travel pump 50B (travelmotor 36R) can operate smoothly.

For descriptive purposes, as for the operation members 54A and 54B, onedirection is referred to as “forward” (“F” in FIG. 1), and the otherdirection opposite to the one direction is referred to as “backward”(“B” in FIG. 1). The operation valves (operation valves 46A and 46C)disposed on a side, which respectively operate when the operationmembers 54A and 54B are operated in the one direction, are referred toas “first operation valves”, and the operation valves (operation valves46B and 46D) disposed on the other side, which respectively operate whenthe operation members 54A and 54B are respectively operated in the otherdirection, are referred to as “second operation valves”.

The oil paths 45 a and 45 c respectively coupling the first pressurereceiving portions 51 a and 52 a and the first operation valves 46A and46C are referred to as “first pilot oil paths”. The oil paths 45 b and45 d respectively coupling the second pressure receiving portions 51 band 52 b and the second operation valves 46B and 46D are referred to as“second pilot oil paths”. The drain oil paths 91 a and 91 c respectivelycoupled to the first pilot oil paths 45 a and 45 c are referred to as“first drain oil paths”. The drain oil paths 91 b and 91 d respectivelycoupled to the second pilot oil paths 45 b and 45 d are referred to as“second drain oil paths”. The alteration portions 94 a and 94 crespectively disposed in the first drain oil paths 91 a and 91 c arereferred to as “first alteration portions”. The alteration portions 94 band 94 d respectively disposed in the second drain oil paths 91 b and 91d are referred to as “second alteration portions”. The differentialpressure generation portions 95A and 95C respectively disposed in thefirst drain oil paths 91 a and 91 c are referred to as “firstdifferential pressure generation portions”. The differential pressuregeneration portions 95B and 95D respectively disposed in the seconddrain oil paths 91 b and 91 d are referred to as “second differentialpressure generation portions”.

As illustrated in FIG. 1, when focused on the first pressure receivingportion 51 a of the travel pump 50A, the drain oil path 91 a is disposedin the first pilot oil path 45 a coupling the first pressure receivingportion 51 a and the first operation valve 46A, and the first alterationportion 94 a and the first differential pressure generation portion 95Aare disposed in the first drain oil path 91 a. When focused on thesecond pressure receiving portion 51 b of the travel pump 50A, thesecond drain oil path 91 b is disposed in the second pilot oil path 45 bcoupling the second pressure receiving portion 51 b and the secondoperation valve 46B, and the second alteration portion 94 b and thesecond differential pressure generation portion 95B are disposed in thesecond drain oil path 91 b.

Therefore, when the operation member 54A is swung forward (in the onedirection) or backward (in the other direction) from a neutral position,within a predetermined range (e.g., dead zone) from the neutralposition, the first differential pressure generation portion 95A or thesecond differential pressure generation portion 95B can limit the pilotoil to be drained from the first alteration portion 94 a or the secondalteration portion 94 b. As a result, in an initial stage of theoperation of the operation member 54A, first pressure representing thepilot pressure acting onto the first pressure receiving portion 51 a ofthe travel pump 50A or second pressure representing the pilot pressureacting onto the second pressure receiving portion 51 b can be raised. Onthe other hand, when the operation member 54A is swung forward orbackward exceeding the predetermined range, the pilot oil drains fromthe first alteration portion 94 a and the first differential pressuregeneration portion 95A or the second alteration portion 94 b and thesecond differential pressure generation portion 95B. Therefore, an upperlimit can be set for the secondary pilot pressure acting onto the travelpump 50A.

When focused on the first pressure receiving portion 52 a of the travelpump 50B, the first drain oil path 91 c is disposed in the first pilotoil path 45 c coupling the first pressure receiving portion 52 a and thefirst operation valve 46C, and the alteration portion 94 c and the firstdifferential pressure generation portion 95C are disposed in the firstdrain oil path 91 c. When focused on the second pressure receivingportion 52 b of the travel pump 50A, the second drain oil path 91 d isdisposed in the second pilot oil path 45 d coupling the second pressurereceiving portion 52 b and the second operation valve 46D, and thealteration portion 94 d and the second differential pressure generationportion 95D are disposed in the second drain oil path 91 d.

Therefore, when the operation member 54B is swung forward (in the onedirection) or backward (in the other direction) from a neutral position,within the predetermined range from the neutral position, the firstdifferential pressure generation portion 95C or the second differentialpressure generation portion 95D can limit the pilot oil to be drainedfrom the first alteration portion 94 c or the second alteration portion94 d. As a result, in an initial stage of the operation of the operationmember 54B, first pressure representing the pilot pressure acting ontothe first pressure receiving portion 52 a of the travel pump 50B orsecond pressure representing the pilot pressure acting onto the secondpressure receiving portion 52 b can be raised. On the other hand, whenthe operation member 54B is swung forward or backward exceeding thepredetermined range, the pilot oil drains from the first alterationportion 94 c and the first differential pressure generation portion 95Cor the second alteration portion 94 d and the second differentialpressure generation portion 95D. Therefore, an upper limit can be setfor the secondary pilot pressure acting onto the travel pump 50B.

FIGS. 2A and 2B are graphs each illustrating a relationship between anoperation amount of an operation member (operation valve) and pilotpressure. The pilot pressure illustrated in FIGS. 2A and 2B representsthe secondary pilot pressure (secondary pilot pressure acting onto thepressure receiving portions) in the first pilot oil paths 45 a and 45 cand the second pilot oil paths 45 b and 45 d. The pilot pressureillustrated in FIGS. 2A and 2B represents the pressure when throttleareas of the throttle portions 93 a, 93 b, 93 c, and 93 d respectivelydisposed in the plurality of pilot oil paths 45 and throttle areas ofthe alteration portions 94 a, 94 b, 94 c, and 94 c respectively disposedin the drain oil paths 91 are made identical to each other, and when anoperation member 93 is operated. The pressure illustrated in FIGS. 2Aand 2B is merely an example. The embodiment of the present invention isnot limited to this pressure.

As illustrated in FIGS. 2A and 2B, when the secondary pilot pressure isnot limited (when the pilot oil in the first pilot oil paths 45 a and 45c and the second pilot oil paths 45 b and 45 d is not drained to thedrain oil paths 91), pilot pressure (a target pilot pressure/anadditional target pilot pressure) P1 reaches pilot pressure P3 requiredfor operating the hydraulic actuators upon an operation amount exceeding20%, switching control valves 20 and allowing the hydraulic actuators tostart operating. After the operation amount exceeds approximately 20%,the pilot pressure P1 raises (increases) the secondary pilot pressure inaccordance with the operation amount.

As can be seen conventionally, when an anti-stall event occurs to limitprimary pilot pressure of pilot oil to be supplied to the operationvalves, as illustrated in FIG. 2A, pilot pressure P4 reaches pilotpressure P3 upon an operation amount exceeding approximately 20%,switching the control valves 20 and allowing the hydraulic actuators tostart operating. When primary pilot pressure is limited, as can be seenconventionally, upon an operation amount exceeding 30%, the secondarypilot pressure P4 becomes constant, lowering ease of operation.

As illustrated in FIG. 2B, when only the drain oil paths 91 and thealteration portions 94 are simply disposed in order to limit secondarypilot pressure (when the first differential pressure generation portions95A and 95C and the second differential pressure generation portions 95Band 95D are not disposed), and when either of the operation members 54Aand 54B is operated, neither pilot pressure P2 (P2 is an example of anadjusted target pilot pressure/an adjusted additional target pilotpressure) reaches pilot pressure P3, nor the control valves 20 switch,unless an operation amount exceeds approximately 70%. Therefore, ease ofoperation could lower.

As described above in the embodiment, in the hydraulic system in whichthe first differential pressure generation portions 95A and 95C and thesecond differential pressure generation portions 95B and 95D aredisposed, when either of the operation members 54A and 54B is operated,as illustrated in FIG. 2B, secondary pilot pressure P5 can becomeapproximately equal to or can exceed the pilot pressure P3 upon theoperation amount exceeding approximately 30%, allowing the travel pumps50 to supply the pilot oil and allowing the travel motors 36 and 36R tooperate at a level almost identical to a normal level. It is preferablethat differential pressure in the first differential pressure generationportions 95A and 95C and the second differential pressure generationportions 95B and 95D is set to pressure, i.e., P3, at which the travelpumps 50 start operating and a regulator operates.

As described above in the embodiment, the pilot pressure P5 can bealtered with respect to an operation amount with the alteration portions94 respectively by means of variable throttle valves, for example. Forexample, as illustrated in FIG. 2B, with the alteration portions 94, thegradient of the pilot pressure P5 (P5 is an example of an adjustedtarget pilot pressure/an adjusted additional target pilot pressure) canbe altered. Therefore, even when an operation amount is within apredetermined range, secondary pilot pressure can be altered, achievingease of operation in conformity to various situations.

FIGS. 3A to 3D illustrate modifications in which, in the hydraulicsystem of the traveling system, drain oil paths, throttle portions, anddifferential pressure generation portions, for example, are modified.Although FIG. 3A illustrates an example with modified drain oil paths,throttle portions, and differential pressure generation portions aroundthe travel pump 50A, the example can be applied to another travel pump,such as the travel pump 50B. In FIG. 3A, other portions are configuredidentical to the hydraulic system according to the above describedembodiment.

As illustrated in FIG. 3A, a drain oil path 47 includes a first couplingoil path 48 and a second coupling oil path 49. The first coupling oilpath 48 is an oil path coupled to the first pilot oil path 45 a and thesecond pilot oil path 45 b. The second coupling oil path 49 is coupledto the first coupling oil path 48 to drain the hydraulic oil in thefirst coupling oil path 48 to a hydraulic oil tank, for example. In thesecond coupling oil path 49, a throttle portion 101 with a fixedthrottle area (inner diameter) is disposed. A differential pressuregeneration portion 95E is disposed downstream relative to the throttleportion 101. The differential pressure generation portion 95E is a checkvalve configured to permit the hydraulic oil to drain from the firstcoupling oil path 48 via the second coupling oil path 49, and to preventthe hydraulic oil from flowing from the second coupling oil path 49, viathe throttle portion 101, to the first coupling oil path 48.

In the first coupling oil path 48, a first check valve 102 a and asecond check valve 102 b are disposed. The first check valve 102 a isdisposed between a coupling portion 105 coupling the first coupling oilpath 48 and the second coupling oil path 49 and a coupling portion 106 acoupling the first coupling oil path 48 and the first pilot oil path 45a. The first check valve 102 a is configured to permit the hydraulic oilto flow from the first pilot oil path 45 a to the second coupling oilpath 49, and to prevent the hydraulic oil from flowing from the secondcoupling oil path 49 to the first pilot oil path 45 a.

The second check valve 102 b is disposed between the coupling portion105 and a coupling portion 106 b coupling the first coupling oil path 48and the second pilot oil path 45 b. The second check valve 102 b isconfigured to permit the hydraulic oil to flow from the second pilot oilpath 45 b to the second coupling oil path 49, and to prevent thehydraulic oil from flowing from the second coupling oil path 49 to thesecond pilot oil path 45 b.

In the first coupling oil path 48, between the differential pressuregeneration portion 95E and the coupling portion 105, a switching valve107 is disposed. The switching valve 107 is a valve configured to switchbetween a first position 107 a that is an open position and a secondposition 107 b that is a closed position. The switching valve 107 isswitched by a switch 167 that is coupled to the controller 165, and thatis configured to switch ON or OFF. The switch 167 is disposed adjacentto the driver's seat 8 so as to be operated by a driver, for example.When a driver operates the switch 167 to turn ON the switch 167, thecontroller 165 switches the switching valve 107 to the second position107 b. This can limit the secondary pilot pressure. On the other hand,when the driver operates the switch 167 to turn OFF the switch 167, thecontroller 165 switches the switching valve 107 to the first position107 a. This can stop limiting of the secondary pilot pressure.

As illustrated in FIG. 3B, in the first coupling oil path 48, a pressureselection valve 108 is disposed. The pressure selection valve 78 isconfigured to transmit, to the second coupling oil path 49, pilotpressure in the first pilot oil path 45 a or the second pilot oil path45 b, in other words, pilot pressure acting onto the first pilot oilpath 45 a (in the first coupling oil path 48, between the couplingportion 106 a and the pressure selection valve 108) or pilot pressureacting onto the second pilot oil path 45 b (in the first coupling oilpath 48, between the coupling portion 106 b and the pressure selectionvalve 108), whichever higher.

In the second coupling oil path 49, an alteration portion 94 e includinga variable throttle portion is disposed. A differential pressuregeneration portion 95F including a check valve is disposed downstreamrelative to the alteration portion 94 e.

As illustrated in FIG. 3C, in the second coupling oil path 49, analteration portion 94 f is disposed. The alteration portion 94 f is amanual type variable throttle valve. A differential pressure generationportion 95G including a relief valve is disposed downstream relative tothe alteration portion 94 f.

As illustrated in FIG. 3D, the drain oil paths 91 include a plurality ofdrain oil paths 91 a, 91 b, 91 c, 91 c, 91 d, and 91 e. The drain oilpath 91 e is an oil path joining downstream sides of the other oil pathsthat are the drain oil paths 91 a, 91 b, 91 c, and 91 d. In the drainoil path 91 e, a differential pressure generation portion 95H includinga check valve is disposed.

As illustrated in FIG. 5, the hydraulic system includes the firsthydraulic pump Pu1 and the second hydraulic pump Pu2. The firsthydraulic pump Pu1 is used to drive a hydraulic actuator of the boomcylinders 14, the bucket cylinders 15, or an attachment attached to theleading ends of the booms 22. The second hydraulic pump Pu2 (pilot pump)is mainly used to supply pressure of the hydraulic oil, which is to beused as control pressure or signal pressure. Hereinafter, fordescriptive purposes, the hydraulic oil for supplying control pressureor signal pressure will be referred to as “pilot oil”, while pressure ofthe pilot oil will be referred to as “pilot pressure”.

The hydraulic system of the work system includes the plurality ofcontrol valves 20. The plurality of control valves 20 are valvesrespectively configured to control hydraulic actuators (work hydraulicactuators) in the work system. Specifically, the plurality of controlvalves 20 include a boom control valve 20A, a bucket control valve 20B,and an auxiliary control valve 20C. The first hydraulic pump Pu1 and theplurality of control valves 20 (boom control valve 20A, bucket controlvalve 20B, and auxiliary control valve 20C) are coupled with supply oilpaths.

The boom control valve 20A is a valve configured to control the boomcylinders 14 that are work hydraulic actuators configured to operate thebooms 10. The boom control valve 20A is a directly-operated spool typethree-position switching valve. The boom control valve 20A includes aplurality of pressure receiving portions configured to receive pressureof the pilot oil. The plurality of pressure receiving portions include afirst pressure receiving portion 31 a and a second pressure receivingportion 31 b. The first pressure receiving portion 31 a is disposed in amain body (body) of the boom control valve 20A, and is a portion towhich the pilot pressure acts when the pilot oil is supplied. The secondpressure receiving portion 31 b is disposed in the main body of the boomcontrol valve 20A, and is a portion to which the pilot pressure actswhen the pilot oil is supplied.

The bucket control valve 20B is a valve configured to control the bucketcylinders 15 that are work hydraulic actuators configured to operate thebucket 11. The bucket control valve 20B is a pilot-type,directly-operated spool type three-position switching valve. The bucketcontrol valve 20B includes a plurality of pressure receiving portionsconfigured to receive pressure of the pilot oil. The plurality ofpressure receiving portions include a first pressure receiving portion32 a and a second pressure receiving portion 32 b. The first pressurereceiving portion 32 a is disposed in a main body (body) of the bucketcontrol valve 20B, and is a portion to which the pilot pressure actswhen the pilot oil is supplied. The second pressure receiving portion 32b is disposed in the main body of the bucket control valve 20B, and is aportion to which the pilot pressure acts when the pilot oil is supplied.

The auxiliary control valve 20C is a valve configured to controlauxiliary hydraulic actuators (including a hydraulic cylinder and ahydraulic motor) that are work hydraulic actuators configured to operatean auxiliary attachment. The auxiliary control valve 20C is apilot-type, directly-operated spool type three-position switching valve.The auxiliary control valve 20C includes a plurality of pressurereceiving portions configured to receive pressure of the pilot oil. Theplurality of pressure receiving portions include a first pressurereceiving portion 33 a and a second pressure receiving portion 33 b. Thefirst pressure receiving portion 33 a is disposed in a main body (body)of the auxiliary control valve 20C, and is a portion to which the pilotpressure acts when the pilot oil is supplied. The second pressurereceiving portion 33 b is disposed in the main body of the auxiliarycontrol valve 20C, and is a portion to which the pilot pressure actswhen the pilot oil is supplied.

The hydraulic system of the work system includes a work operation device57. The work operation device 57 is a device configured to operate eachof the plurality of control valves 20. When operated, the work operationdevice 57 can set pilot pressure acting onto the pressure receivingportions (first pressure receiving portions 31 a and 32 a and secondpressure receiving portion 31 b and 32 b) of the control valves 20.

Specifically, the work operation device 57 includes an operation member58, and a plurality of operation valves 59A, 59B, 59C, and 59D. Theoperation member 58 is formed into a lever, for example, and isconfigured to swing forward, backward, rightward, and leftward. Theplurality of operation valves 59A, 59B, 59C, and 59D are coupled, viaoil paths, to the second hydraulic pump Pu2, and are each configured toset pilot pressure in accordance with an operation of the operationmember 58.

For example, when the operation member 58 is tilted forward, theoperation valve 59A sets pilot pressure acting onto the first pressurereceiving portion 31 a in accordance with an operation amount of theoperation member 58. When the pilot pressure acting onto the firstpressure receiving portion 31 a is equal to or above a predeterminedvalue, the boom control valve 20A switches, the boom cylinders 14contract, and the booms 10 lower.

When the operation member 58 is tilted backward, the operation valve 59Bsets pilot pressure acting onto the second pressure receiving portion 31b in accordance with an operation amount of the operation member 58.When the pilot pressure acting onto the second pressure receivingportion 31 b is equal to or above a predetermined value, the boomcontrol valve 20A switches, the boom cylinders 14 extend, and the booms10 rise.

When the operation member 58 is tilted rightward, the operation valve59C sets pilot pressure acting onto the first pressure receiving portion32 a in accordance with an operation amount of the operation member 58.When the pilot pressure acting onto the first pressure receiving portion32 a is equal to or above a predetermined value, the bucket controlvalve 20B switches, the bucket cylinders 15 extend, and the bucket 11performs a dump operation. When the operation member 58 is tiltedleftward, the operation valve 59D sets pilot pressure acting onto thesecond pressure receiving portion 32 b in accordance with an operationamount of the operation member 58. When the pilot pressure acting ontothe second pressure receiving portion 32 b is equal to or above apredetermined value, the bucket control valve 20B switches, the bucketcylinders 15 contract, and the bucket 11 performs a scoop operation.

The work operation device 57 includes a switch 55 in addition to theoperation member 58 that is a lever, for example. The switch 55 is aslide switch configured to swing, for example. The switch 55 is coupledto a controller 65. By operating the switch 55, pilot pressure actingonto the pressure receiving portions (first pressure receiving portion33 a and second pressure receiving portion 33 b) of the auxiliarycontrol valve 20C can be set. Specifically, electromagnetic proportionalvalves 56A and 56B are coupled to the controller 65. When the switch 55is swung in one direction, such as leftward, the electromagneticproportional valve 56A changes its degree of opening in accordance withan operation amount of the switch 55. When the switch 55 is swung inanother direction, such as rightward, the electromagnetic proportionalvalve 56B changes its degree of opening in accordance with an operationamount of the switch 55. As described above, by setting theelectromagnetic proportional valves 55A and 55B via the switch 55, thepilot pressure acting onto the first pressure receiving portion 33 a andthe second pressure receiving portion 33 b of the auxiliary controlvalve 20C can be altered. Therefore, the auxiliary control valve 20C canbe switched.

The hydraulic system of the work system includes a plurality of pilotoil paths 60. The plurality of pilot oil paths 60 are oil pathsrespectively coupling the work operation device 57 and the plurality ofpressure receiving portions (first pressure receiving portions 31 a and32 a and second pressure receiving portions 31 b and 32 b).Specifically, the plurality of pilot oil paths 60 include a pilot oilpath 60 a coupling the operation valve 59A and the first pressurereceiving portion 31 a, a pilot oil path 60 b coupling the operationvalve 59B and the second pressure receiving portion 31 b, a pilot oilpath 60 c coupling the operation valve 59C and the first pressurereceiving portion 32 a, and a pilot oil path 60 d coupling the operationvalve 59D and the second pressure receiving portion 32 b.

The hydraulic system of the work system includes a plurality of drainoil paths 70. The plurality of drain oil paths 70 are oil pathsrespectively coupled to the plurality of pilot oil paths 60, and areconfigured to drain the pilot oil in the plurality of pilot oil paths 60to the hydraulic oil tank 23, for example. Specifically, the pluralityof drain oil paths 70 include a drain oil path 70 a coupled to the pilotoil path 60 a, a drain oil path 70 b coupled to the pilot oil path 60 b,a drain oil path 70 c coupled to the pilot oil path 60 c, and a drainoil path 70 d coupled to the pilot oil path 60 d.

In the plurality of pilot oil paths 60, throttle portions 84 a, 84 b, 84c, and 84 d are respectively disposed between the plurality of operationvalves 59A, 59B, 59C, and 59D and coupling portions 79 a, 79 b, 79 c,and 79 d respectively coupling the plurality of pilot oil paths 60 andthe plurality of drain oil paths 70.

The hydraulic system of the work system includes a plurality ofalteration portions (pressure adjustment valves) 80, and a plurality ofdifferential pressure generation portions (differential pressuregeneration valves) 90. The plurality of alteration portions 80 are eachconfigured to alter the pilot pressure (secondary pilot pressure) actingonto the pressure receiving portions (first pressure receiving portion33 a and second pressure receiving portion 33 b) with respect to anoperation amount of the work operation device 57, for example, when aload of the engine 9 is higher than a load threshold. In other words,the plurality of alteration portions 80 are each configured to at leastalter an increasing rate (gradient) of the secondary pilot pressure withrespect to an operation amount of the work operation device 57. Theplurality of alteration portions 80 may each be configured to alter adecreasing rate (gradient) of the secondary pilot pressure with respectto an operation amount of the work operation device 57.

The plurality of alteration portions 80 are respectively disposed in theplurality of drain oil paths 70. The plurality of alteration portions 80include an alteration portion 80 a disposed in the drain oil path 70 a,an alteration portion 80 b disposed in the drain oil path 70 b, analteration portion 80 c disposed in the drain oil path 70 c, and analteration portion 80 d disposed in the drain oil path 70 d.

The alteration portions 80 a, 80 b, 80 c, and 80 d are electromagneticvariable throttle valves (electric variable throttle valves). Thevariable throttle valves are valves each configured to switch between aclosed position 81, at which a throttle area is reduced to zero, thatis, the path is fully closed, and an open position 82, at which the pathis fully open. The variable throttle valves are each configured to set athrottle area at a desired position between the closed position 81 andthe open position 82.

A throttle amount of each of the variable throttle valves can beadjusted with a switch 66 disposed on the controller 65, for example.The switch 66 is disposed adjacent to the driver's seat 8 so as to beoperated by a driver, for example. For example, the switch 66 is aswitch configured to switch between ON and OFF. A signal indicative ofON or OFF of the switch 66 then is input to the controller 65. When theswitch 66 is in ON, the pilot pressure in the plurality of pilot oilpaths 60 (secondary pilot pressure in the operation valves 59A, 59B,59C, and 59D) is limited, while, when the switch 66 is in OFF, nosecondary pilot pressure is limited. For example, when the switch 66 isin ON, the controller 65 sets throttle amounts for the variable throttlevalves so that throttle diameters (throttle areas) of the alterationportions 80 and throttle diameters of the throttle portions 84 a, 84 b,84 c, and 84 d respectively match each other. On the other hand, whenthe switch 66 is in OFF, the controller 65 causes the variable throttlevalves to each move to the closed position 81. By disposing analteration operation member 68, which is a rotatable volume or a slideswitch, for example, on the controller 65, and by operating thealteration operation member 68, an increasing rate (gradient) ofsecondary pilot pressure may be altered with respect to an operationamount of the work operation device 57. The above described throttleamounts set in the variable throttle valves are merely examples. Theembodiment of the present invention is not limited to the throttleamounts.

The plurality of differential pressure generation portions 90 arerespectively disposed in the plurality of drain oil paths 70. Theplurality of differential pressure generation portions 90 are portionseach configured to at least generate differential pressure upstream ordownstream relative to the plurality of alteration portions 80.

The plurality of differential pressure generation portions 90 arerespectively disposed downstream relative to the plurality of alterationportions 80. The plurality of differential pressure generation portions90 include a differential pressure generation portion 90A disposeddownstream relative to the alteration portion 80 a, a differentialpressure generation portion 90B disposed downstream relative to thealteration portion 80 b, a differential pressure generation portion 90Cdisposed downstream relative to the alteration portion 80 c, and adifferential pressure generation portion 90D disposed downstreamrelative to the alteration portion 80 d. The differential pressuregeneration portions 90A, 90B, 90C, and 90D are check valves eachconfigured to permit the hydraulic oil to flow from the pilot oil paths60 to the alteration portions 80, and to prevent the hydraulic oil fromflowing to the pilot oil paths 60. Although, in the embodiment, theplurality of differential pressure generation portions 90 arerespectively disposed downstream relative to the plurality of alterationportions 80, the plurality of differential pressure generation portions90 may respectively be disposed upstream relative to the plurality ofalteration portions 80.

In the hydraulic system of the work system, when the operation member 58is swung forward or backward, the control valves 20 (work hydraulicactuators) can operate smoothly. Alternatively, when the operationmember 58 is swung rightward or leftward, the control valves 20 (workhydraulic actuators) can operate smoothly.

For descriptive purposes, as for the operation member 58, one directionis referred to as “front or right”, and the other direction opposite tothe one direction is referred to as “back or left”. The operation valves(operation valves 59A and 59C) disposed on a side, which respectivelyoperate when the operation member 58 is operated in the one direction,are referred to as “first operation valves”, and the operation valves(operation valves 59B and 59D) disposed on the other side, whichrespectively operate when the operation member 58 is operated in theother direction, are referred to as “second operation valves”.

The oil paths 60 a and 60 c respectively coupling the first pressurereceiving portions 31 a and 32 a and the first operation valves 59A and59C are referred to as “first pilot oil paths”. The oil paths 60 b and60 d respectively coupling the second pressure receiving portions 31 band 32 b and the second operation valves 59B and 59D are referred to as“second pilot oil paths”. The drain oil paths 70 a and 70 c respectivelycoupled to the first pilot oil paths 60 a and 60 c are referred to as“first drain oil paths”. The drain oil paths 70 b and 70 d respectivelycoupled to the second pilot oil paths 60 b and 60 d are referred to as“second drain oil paths”. The alteration portions 80 a and 80 crespectively disposed in the first drain oil paths 70 a and 70 c arereferred to as “first alteration portions”. The alteration portions 80 band 80 d respectively disposed in the second drain oil paths 70 b and 70d are referred to as “second alteration portions”. The differentialpressure generation portions 90A and 90C respectively disposed in thefirst drain oil paths 70 a and 70 c are referred to as “firstdifferential pressure generation portions”. The differential pressuregeneration portions 90B and 90D respectively disposed in the seconddrain oil paths 70 b and 70 d are referred to as “second differentialpressure generation portions”.

As illustrated in FIG. 5, when focused on the first pressure receivingportion 31 a of the boom control valve 20A, the drain oil path 70 a isdisposed in the first pilot oil path 60 a coupling the first pressurereceiving portion 31 a and the first operation valve 59A, and the firstalteration portion 80 a and the first differential pressure generationportion 90A are disposed in the first drain oil path 70 a. When focusedon the second pressure receiving portion 31 b of the boom control valve20A, the second drain oil path 70 b is disposed in the second pilot oilpath 60 b coupling the second pressure receiving portion 31 b and thesecond operation valve 59B, and the second alteration portion 80 b andthe second differential pressure generation portion 90B are disposed inthe second drain oil path 70 b.

Therefore, when the operation member 58 is swung forward (in the onedirection; “F” in FIG. 5) or backward (in the other direction; “B” inFIG. 5) from a neutral position, within a predetermined range (e.g.,dead zone) from the neutral position, the first differential pressuregeneration portion 90A or the second differential pressure generationportion 90B can limit the pilot oil to be drained from the firstalteration portion 80 a or the second alteration portion 80 b. As aresult, in an initial stage of the operation of the operation member 58,first pressure representing the pilot pressure acting onto the firstpressure receiving portion 31 a of the boom control valve 20A or secondpressure representing the pilot pressure acting onto the second pressurereceiving portion 31 b can be raised. On the other hand, when theoperation member 58 is swung forward or backward exceeding thepredetermined range, the pilot oil drains from the first alterationportion 80 a and the first differential pressure generation portion 90Aor the second alteration portion 80 b and the second differentialpressure generation portion 90B. Therefore, an upper limit can be setfor the secondary pilot pressure acting onto the boom control valve 20A.

When focused on the first pressure receiving portion 32 a of the bucketcontrol valve 20B, the first drain oil path 70 c is disposed in thefirst pilot oil path 60 c coupling the first pressure receiving portion32 a and the first operation valve 59C, and the alteration portion 80 cand the first differential pressure generation portion 90C are disposedin the first drain oil path 70 c. When focused on the second pressurereceiving portion 32 b of the boom control valve 20A, the second drainoil path 70 d is disposed in the second pilot oil path 60 d coupling thesecond pressure receiving portion 32 b and the second operation valve59D, and the alteration portion 80 d and the second differentialpressure generation portion 90D are disposed in the second drain oilpath 70 d.

Therefore, when the operation member 58 is swung rightward (in the onedirection; “R” in FIG. 5) or leftward (in the other direction; “L” inFIG. 5) from the neutral position, within a predetermined range from theneutral position, the first differential pressure generation portion 90Cor the second differential pressure generation portion 90D can limit thepilot oil to be drained from the first alteration portion 80 c or thesecond alteration portion 80 d. As a result, in an initial stage of theoperation of the operation member 58, first pressure representing thepilot pressure acting onto the first pressure receiving portion 32 a ofthe bucket control valve 20B or second pressure representing the pilotpressure acting onto the second pressure receiving portion 32 b can beraised. On the other hand, when the operation member 58 is swungrightward or leftward exceeding the predetermined range, the pilot oildrains from the first alteration portion 80 c and the first differentialpressure generation portion 90C or the second alteration portion 80 dand the second differential pressure generation portion 90D. Therefore,an upper limit can be set for the secondary pilot pressure acting ontothe bucket control valve 20B.

FIGS. 6A and 2B are graphs each illustrating a relationship between anoperation amount of an operation member (operation valve) and pilotpressure. The pilot pressure illustrated in FIGS. 6A and 2B representsthe secondary pilot pressure (secondary pilot pressure acting onto thepressure receiving portions) in the first pilot oil paths 60 a and 60 cand the second pilot oil paths 60 b and 60 d. The pilot pressureillustrated in FIGS. 6A and 2B represents the pressure when throttleareas of the throttle portions 84 a, 84 b, 84 c, and 84 d respectivelydisposed in the plurality of pilot oil paths 60 and throttle areas ofthe alteration portions 80 a, 80 b, 80 c, and 80 c respectively disposedin the drain oil path 70 are made identical to each other, and when anoperation member 84 is operated. The pressure illustrated in FIGS. 6Aand 2B is merely an example. The embodiment of the present invention isnot limited to this pressure.

As illustrated in FIG. 6A, when the secondary pilot pressure is notlimited (when the pilot oil in the first pilot oil paths 60 a and 60 cand the second pilot oil paths 60 b and 60 d is not drained to the drainoil paths 70), pilot pressure P1 reaches pilot pressure P3 required foroperating the hydraulic actuators upon an operation amount exceeding20%, switching the control valves 20 and allowing the hydraulicactuators to start operating. After the operation amount exceedsapproximately 20%, the pilot pressure P1 raises (increases) thesecondary pilot pressure in accordance with the operation amount.

As can be seen conventionally, when an anti-stall event occurs to limitprimary pilot pressure of pilot oil to be supplied to the operationvalves, as illustrated in FIG. 6A, pilot pressure P4 reaches pilotpressure P3 upon an operation amount exceeding approximately 20%,switching the control valves 20 and allowing the hydraulic actuators tostart operating. When primary pilot pressure is limited, as can be seenconventionally, upon an operation amount exceeding 30%, the secondarypilot pressure P4 becomes constant, lowering ease of operation.

As illustrated in FIG. 6B, when only the drain oil paths 70 and thealteration portions 80 are simply disposed in order to limit secondarypilot pressure (when the first differential pressure generation portions90A and 90C and the second differential pressure generation portions 90Band 90D are not disposed), and when the operation member 58 is operated,neither pilot pressure P2 reaches pilot pressure P3, nor the controlvalves 20 switch, unless an operation amount exceeds approximately 70%.Therefore, ease of operation could lower.

On the other hand, as described above in the embodiment, in thehydraulic system in which the first differential pressure generationportions 90A and 90C and the second differential pressure generationportions 90B and 90D are disposed, when the operation member 58 isoperated, as illustrated in FIG. 6B, secondary pilot pressure P5 canbecome approximately equal to or can exceed the pilot pressure P3 uponan operation amount exceeding approximately 30%, switching the controlvalves 20 and allowing the hydraulic actuators to operate at a levelalmost identical to a normal level. It is preferable that differentialpressure in the first differential pressure generation portions 90A and90C and the second differential pressure generation portions 90B and 90Dis set to pilot pressure P3, i.e., pressure at which the control valves20 start switching from the neutral position and the spools move.

As described above in the embodiment, the pilot pressure P5 can bealtered with respect to an operation amount with the alteration portions80 respectively by means of variable throttle valves, for example. Forexample, as illustrated in FIG. 6B, with the alteration portions 80, thegradient of the pilot pressure P5 can be altered. Therefore, even whenan operation amount is within a predetermined range, secondary pilotpressure can be altered, achieving ease of operation in conformity tovarious situations.

FIGS. 7A to 7D illustrate modifications in which, in the hydraulicsystem of the work system, drain oil paths, throttle portions, anddifferential pressure generation portions, for example, are modified.Although FIG. 7A illustrates an example with modified drain oil paths,throttle portions, and differential pressure generation portions aroundthe boom control valve 20A, the example can be applied to anothercontrol valve. In FIG. 7A, other portions are configured identical tothe hydraulic system according to the above described embodiment.

As illustrated in FIG. 7A, a drain oil path 71 includes a first couplingoil path 72 and a second coupling oil path 73. The first coupling oilpath 72 is an oil path coupled to the first pilot oil path 60 a and thesecond pilot oil path 60 b. The second coupling oil path 73 is coupledto the first coupling oil path 72 to drain the hydraulic oil in thefirst coupling oil path 72 to a hydraulic oil tank, for example. In thesecond coupling oil path 73, a throttle portion 83 with a fixed throttlearea (inner diameter) is disposed. A differential pressure generationportion 90E is disposed downstream relative to the throttle portion 83.The differential pressure generation portion 90E is a check valveconfigured to permit the hydraulic oil to drain from the first couplingoil path 72 via the second coupling oil path 73, and to prevent thehydraulic oil from flowing from the second coupling oil path 73, via thethrottle portion 83, to the first coupling oil path 72.

In the first coupling oil path 72, a first check valve 74 a and a secondcheck valve 74 b are disposed. The first check valve 74 a is disposedbetween a coupling portion 75 coupling the first coupling oil path 72and the second coupling oil path 73 and a coupling portion 76 a couplingthe first coupling oil path 72 and the first pilot oil path 60 a. Thefirst check valve 74 a is configured to permit the hydraulic oil to flowfrom the first pilot oil path 60 a to the second coupling oil path 73,and to prevent the hydraulic oil from flowing from the second couplingoil path 73 to the first pilot oil path 60 a.

The second check valve 74 b is disposed between the coupling portion 75and a coupling portion 76 b coupling the first coupling oil path 72 andthe second pilot oil path 60 b. The second check valve 74 b isconfigured to permit the hydraulic oil to flow from the second pilot oilpath 60 b to the second coupling oil path 73, and to prevent thehydraulic oil from flowing from the second coupling oil path 73 to thesecond pilot oil path 60 b.

In the first coupling oil path 72, between the differential pressuregeneration portion 90E and the coupling portion 75, a switching valve 77is disposed. The switching valve 77 is a valve configured to switchbetween a first position 77 a that is a closed position and a secondposition 77 b that is an open position. The switching valve 77 isswitched by a switch 67 that is coupled to the controller 65, and thatis configured to switch ON or OFF. The switch 67 is disposed adjacent tothe driver's seat 8 so as to be operated by a driver, for example. Whena driver operates the switch 67 to turn ON the switch 67, the controller65 switches the switching valve 77 to the second position 77 b. This canlimit the secondary pilot pressure. On the other hand, when the driveroperates the switch 67 to turn OFF the switch 67, the controller 65switches the switching valve 77 to the first position 77 a. This canstop limiting of the secondary pilot pressure.

As illustrated in FIG. 7B, in the first coupling oil path 72, a pressureselection valve 78 is disposed. The pressure selection valve 78 isconfigured to transmit, to the second coupling oil path 73, pilotpressure in the first pilot oil path 60 a or the second pilot oil path60 b, in other words, pilot pressure acting onto the first pilot oilpath 60 a (in the first coupling oil path 72, between the couplingportion 76 a and the pressure selection valve 78) or pilot pressureacting onto the second pilot oil path 60 b (in the first coupling oilpath 72, between the coupling portion 76 b and the pressure selectionvalve 78), whichever higher.

In the second coupling oil path 73, an alteration portion 80 e includinga variable throttle valve is disposed. A differential pressuregeneration portion 90F including a check valve is disposed downstreamrelative to the alteration portion 80 e.

As illustrated in FIG. 7C, in the second coupling oil path 73, analteration portion 80 f is disposed. The alteration portion 80 f is amanual type variable throttle valve. A differential pressure generationportion 90G including a relief valve is disposed downstream relative tothe alteration portion 80 f.

As illustrated in FIG. 7D, the drain oil paths 70 include a plurality ofdrain oil paths 70 a, 70 b, 70 c, 70 c, 70 d, and 70 e. The drain oilpath 70 e is an oil path joining downstream sides of the other oil pathsthat are the drain oil paths 70 a, 70 b, 70 c, and 70 d. In the drainoil path 70 e, a differential pressure generation portion 90H includinga check valve is disposed.

The embodiments disclosed herein are given only for illustration andshould not be construed as being restrictive. The scope of the presentinvention is defined by the claims, not by the above description, andintended to include all modifications within a gist and a scopeequivalent to those of the claims. The differential pressure generationportions 90 and 95 may respectively be either check valves or reliefvalves.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A hydraulic system in a work machine, comprising:a hydraulic pump to supply pilot hydraulic oil in a hydraulic oilreservoir; a work hydraulic actuator to move a work device of the workmachine; a control valve to control the work hydraulic actuator based ona pilot pressure of the pilot hydraulic oil; a work operation devicethrough which a target pilot pressure of the pilot hydraulic oil isinput to control, according to the target pilot pressure, the pilotpressure supplied from the hydraulic pump to the control valve; a pilotoil path connecting the work operation device and the control valve tosupply the pilot pressure to the control valve; a drain oil path dividedfrom the pilot oil path; and a pressure adjustment valve provided in thedrain oil path to adjust the pilot pressure to be less than the targetpilot pressure when a condition is satisfied, wherein the condition issatisfied when a load of an engine is higher than a load threshold. 2.The hydraulic system according to claim 1, further comprising: adifferential pressure generation valve provided in the drain oil path togenerate differential pressure upstream or downstream relative to thepressure adjustment valve, wherein the pressure adjustment valveincludes a variable throttle valve.
 3. The hydraulic system according toclaim 2, wherein the differential pressure generation valve includes atleast one of a check valve and a relief valve.
 4. The hydraulic systemaccording to claim 1, further comprising: a work hydraulic pump tosupply hydraulic oil to the work hydraulic actuator, wherein the workhydraulic actuator is driven by the hydraulic oil, and wherein thecontrol valve is provided between the work hydraulic actuator and thework hydraulic pump to control a flow of the hydraulic oil between thework hydraulic pump and the work hydraulic actuator.
 5. The hydraulicsystem according to claim 4, comprising: an additional pilot oil pathconnecting the work operation device and the control valve, anadditional pilot pressure of the pilot hydraulic oil being supplied fromthe work operation device to the control valve via the additional pilotoil path, the control being configured to control a flow direction ofthe hydraulic oil supplied from the work hydraulic pump to the workhydraulic actuator based on the pilot pressure and the additional pilotpressure, an additional target pilot pressure of the pilot hydraulic oilbeing input through the work operation device to control the additionalpilot pressure according to the additional target pressure; anadditional drain oil path divided from the additional pilot oil path;and an additional pressure adjustment valve provided in the additionaldrain oil path to adjust the additional pilot pressure to be less thanthe additional target pilot pressure when the condition is satisfied. 6.The hydraulic system according to claim 5, further comprising: adifferential pressure generation valve provided in the drain oil path togenerate differential pressure upstream or downstream relative to thepressure adjustment valve; and an additional differential pressuregeneration valve provided in the additional drain oil path to generateadditional differential pressure upstream or downstream relative to theadditional pressure adjustment valve, wherein the pressure adjustmentvalve and the additional pressure adjustment valve include variablethrottle valves.
 7. The hydraulic system according to claim 6, whereinthe work operation device includes: an operation member configured to beoperated in a first direction or a second direction; a first operationvalve connected to the pilot oil path to supply the pilot pressure ofthe pilot hydraulic oil to the control valve when the operation memberis operated in the first direction; and a second operation valveconnected to the additional pilot oil path to supply the additionalpilot pressure of the pilot hydraulic oil when the operation member isoperated in the second direction.
 8. The hydraulic system according toclaim 7, wherein the control valve includes a pressure receiver and anadditional pressure receiver, wherein the pilot path connects the firstoperation valve and the pressure receiver of the control valve to supplythe pilot pressure to the pressure receiver, and wherein the additionalpilot path connects the second operation valve and the additionalpressure receiver of the control valve to supply the additional pilotpressure to the additional pressure receiver.
 9. The hydraulic systemaccording to claim 4, further comprising: an additional pilot oil pathconnecting the work operation device and the control valve, anadditional pilot pressure of the pilot hydraulic oil being supplied fromthe work operation device to the control valve via the additional pilotoil path, the control valve being configured to control a flow directionof the hydraulic oil supplied from the work hydraulic pump to thecontrol valve based on the pilot pressure and the additional pilotpressure, an additional target pilot pressure of the pilot hydraulic oilbeing input through the work operation device to control the additionalpilot pressure according to the additional target pilot pressure. 10.The hydraulic system according to claim 9, further comprising: adifferential pressure generation valve provided in the drain oil path togenerate differential pressure upstream or downstream relative to thepressure adjustment valve, wherein the pressure adjustment valveincludes a variable throttle valve.
 11. The hydraulic system accordingto claim 10, wherein the work operation device includes: an operationmember configured to be operated in a first direction and a seconddirection; a first operation valve connected to the pilot oil path tosupply the pilot pressure of the pilot hydraulic oil to the controlvalve when the operation member is operated in the first direction; anda second operation valve connected to the additional pilot oil path tosupply the additional pilot pressure of the pilot hydraulic oil when theoperation member is operated in the second direction.
 12. The hydraulicsystem according to claim 11, wherein the control valve includes apressure receiver and an additional pressure receiver, wherein the pilotpath connects the first operation valve and the pressure receiver of thecontrol valve to supply the pilot pressure to the pressure receiver, andwherein the additional pilot path connects the second operation valveand the additional pressure receiver of the control valve to supply theadditional pilot pressure to the additional pressure receiver.
 13. Thehydraulic system according to claim 10, wherein the drain oil pathincludes: a first coupling oil path connecting the pilot oil path andthe additional pilot oil path; and a second coupling oil path dividedfrom the first coupling oil path, wherein the pressure adjustment valveand the differential pressure generation valve are provided in thesecond coupling oil path.
 14. The hydraulic system according to claim13, further comprising: a pressure selection valve provided in the firstcoupling oil path to select a selected pressure of the pilot hydraulicoil from the pilot pressure and the additional pilot pressure to applythe selected pressure to the pilot hydraulic oil in the second couplingoil path.
 15. The hydraulic system according to claim 13, furthercomprising: a first check valve provided in the first coupling oil pathto be opened such that the pilot hydraulic oil flows from the pilot oilpath to the second coupling oil path and to be closed such that thepilot hydraulic oil does not flow from the second coupling oil path tothe pilot oil path; and a second check valve provided in the firstcoupling oil path to be opened such that the hydraulic oil flows fromthe additional pilot oil path to the second coupling oil path and to beclosed such that the pilot hydraulic oil does not flow from the secondcoupling oil path to the additional pilot oil path.
 16. The hydraulicsystem according to claim 1, wherein the control valve includes apressure receiver, and wherein the pilot path connects the workoperation device and the pressure receiver of the control valve tosupply the pilot pressure to the pressure receiver.
 17. The hydraulicsystem according to claim 1, wherein the pressure adjustment valve isconfigured to adjust the pilot pressure such that a first value of thepilot pressure minus a pressure threshold is substantially in directproportion to a second value of the target pilot pressure minus thepressure threshold when the target pilot pressure is more than thepressure threshold.
 18. The hydraulic system according to claim 1,wherein the condition is satisfied when a driver turns on a switchdisposed adjacent to a driver's seat.